CS162Operating Systems andSystems ProgrammingLecture 26Protection and Security in Distributed Systems IIDecember 5, 2007Prof. John Kubiatowiczhttp://inst.eecs.berkeley.edu/~cs162Lec 26.212/05/07Kubiatowicz CS162 ©UCB Fall 2007Review: Authentication: Identifying Users• How to identify users to the system?– Passwords» Shared secret between two parties» Since only user knows password, someone types correct password ⇒ must be user typing it» Very common technique– Smart Cards» Electronics embedded in card capable of providing long passwords or satisfying challenge → response queries» May have display to allow reading of password» Or can be plugged in directly; several credit cards now in this category– Biometrics» Use of one or more intrinsic physical or behavioral traits to identify someone» Examples: fingerprint reader, palm reader, retinal scan» Becoming quite a bit more commonLec 26.312/05/07Kubiatowicz CS162 ©UCB Fall 2007Review: Private Key Cryptography• Private Key (Symmetric) Encryption:– Single key used for both encryption and decryption• Plaintext: Unencrypted Version of message• Ciphertext: Encrypted Version of message• Important properties– Can’t derive plain text from ciphertext (decode) without access to key– Can’t derive key from plain text and ciphertext– As long as password stays secret, get both secrecy and authentication• Symmetric Key Algorithms: DES, Triple-DES, AES InsecureTransmission(ciphertext)DecryptKeyEncryptKeyPlaintextPlaintextSPY CIALec 26.412/05/07Kubiatowicz CS162 ©UCB Fall 2007Goals for Today• Public Encryption• Use of Cryptographic Mechanisms• Authorization Mechanisms• Worms and VirusesNote: Some slides and/or pictures in the following areadapted from slides ©2005 Silberschatz, Galvin, and Gagne Note: Some slides and/or pictures in the following areadapted from slides ©2005 Silberschatz, Galvin, and Gagne. Many slides generated from my lecture notes by Kubiatowicz.Lec 26.512/05/07Kubiatowicz CS162 ©UCB Fall 2007Public Key Encryption• Can we perform key distribution without an authentication server?– Yes. Use a Public-Key Cryptosystem.• Public Key Details– Don’t have one key, have two: Kpublic, Kprivate» Two keys are mathematically related to one another» Really hard to derive Kpublicfrom Kprivateand vice versa– Forward encryption:» Encrypt: (cleartext)Kpublic= ciphertext1» Decrypt: (ciphertext1)Kprivate= cleartext– Reverse encryption:» Encrypt: (cleartext)Kprivate= ciphertext2» Decrypt: (ciphertext2)Kpublic= cleartext– Note that ciphertext1≠ ciphertext2» Can’t derive one from the other!• Public Key Examples:– RSA: Rivest, Shamir, and Adleman» Kpublicof form (kpublic, N), Kprivateof form (kprivate, N)» N = pq. Can break code if know p and q– ECC: Elliptic Curve CryptographyLec 26.612/05/07Kubiatowicz CS162 ©UCB Fall 2007• Idea: Kpubliccan be made public, keep Kprivateprivate• Gives message privacy (restricted receiver):– Public keys (secure destination points) can be acquired by anyone/used by anyone– Only person with private key can decrypt message• What about authentication?– Use combination of private and public key– Alice→Bob: [(I’m Alice)AprivateRest of message]Bpublic– Provides restricted sender and receiver• But: how does Alice know that it was Bob who sent her Bpublic? And vice versa…BprivateAprivatePublic Key Encryption DetailsAlice BobBpublicApublicInsecure ChannelInsecure ChannelLec 26.712/05/07Kubiatowicz CS162 ©UCB Fall 2007Secure Hash Function• Hash Function: Short summary of data (message)– For instance, h1=H(M1) is the hash of message M1» h1fixed length, despite size of message M1.» Often, h1is called the “digest” of M1.• Hash function H is considered secure if – It is infeasible to find M2with h1=H(M2); ie. can’t easily find other message with same digest as given message.– It is infeasible to locate two messages, m1and m2, which “collide”, i.e. for which H(m1) = H(m2)– A small change in a message changes many bits of digest/can’t tell anything about message given its hash• Hash function Examples: MD5, SHA-1, SHA-256DFCD3454BBEA788A751A696C24D97009CA992D17The red foxruns acrossthe iceHashFunctionHashFunction52ED879E70F71D926EB6957008E03CE4CA6945D3FoxLec 26.812/05/07Kubiatowicz CS162 ©UCB Fall 2007Signatures/Certificate Authorities• Can use Xpublicfor person X to define their identity– Presumably they are the only ones who know Xprivate.– Often, we think of Xpublicas a “principle” (user)• Suppose we want X to sign message M?– Use private key to encrypt the digest, i.e. H(M)Xprivate– Send both M and its signature:» Signed message = [M,H(M)Xprivate]– Now, anyone can verify that M was signed by X» Simply decrypt the digest with Xpublic» Verify that result matches H(M)• Now: How do we know that the version of Xpublicthat we have is really from X???– Answer: Certificate Authority» Examples: Verisign, Entrust, Etc.– X goes to organization, presents identifying papers» Organization signs X’s key: [ Xpublic, H(Xpublic)CAprivate]» Called a “Certificate”– Before we use Xpublic, ask X for certificate verifying key» Check that signature over Xpublicproduced by trusted authority• How do we get keys of certificate authority?– Compiled into your browser, for instance!Lec 26.912/05/07Kubiatowicz CS162 ©UCB Fall 2007(pms)Ks• SSL Web Protocol– Port 443: secure http– Use public-key encryption for key-distribution • Server has a certificate signed by certificate authority– Contains server info (organization, IP address, etc)– Also contains server’s public key and expiration date• Establishment of Shared, 48-byte “master secret”– Client sends 28-byte random value ncto server– Server returns its own 28-byte random value ns, plus its certificate certs– Client verifies certificate by checking with public key of certificate authority compiled into browser» Also check expiration date– Client picks 46-byte “premaster” secret (pms), encrypts it with public key of server, and sends to server– Now, both server and client have nc, ns, and pms» Each can compute 48-byte master secret using one-way and collision-resistant function on three values» Random “nonces” ncand nsmake sure master secret freshns,certsSecurity through SSLncLec 26.1012/05/07Kubiatowicz CS162 ©UCB Fall 2007SSL Pitfalls• Netscape claimed to provide secure comm.
View Full Document
Unlocking...